Three types of colored liquid compostions or inks are already known for highlighting pens that serve to highlight text or make it stand out optically.
In the first type, the ink is a traditional colored ink in which the dye(s) is/are at low concentration so as to avoid masking the text which is to be highlighted. Generally such an ink is based on a solvent containing about 70% water and 30% diethylene glycol.
In the second type, concentrated ink is used containing 50% water and 50% fluorescent dye coated in resins that are put into emulsion in water. The concentrated ink is used for making the colored liquid composition for use in the highlighter at a concentration of 20% to 30% by weight, with the reminder of the composition being constituted by a water-retaining agent such as glycerin, sorbitol, diethylene glycol, or triethylene glycol which avoids the tip of the writing instrument drying out too quickly.
In the third type, the colored liquid composition makes use of water-soluble fluorescent dyes and requires the physico-chemical characteristics of the composition to be adapted to obtain a compromise between the fluorescent effect and the properties of the writing instrument in use. By way of example, a colored liquid composition of this type is disclosed in document EP 0 464 304, which composition is based on an aqueous solution of hydroxy-pyrenetrisulfonic acid with basic pH. Such a composition contains 5% to 20% by weight compared with the total weight of liquid of mono-, di-, or trisaccharides obtained by reducing the polyols from which they are derived or by mixing said substances.
Colored liquid compositions for highlighters, in all three of the above-specified types, present the same defect when highlighting text that was previously written using a fountain pen or a roller (ball felt tip) having an ink formulation based on water-soluble dyes. Under such circumstances, it is found that the colored liquid composition of the highlighter moistens the ink of the text and partially dissolves the dyes contained in said ink. As a result the dyes constituting the written text become smeared in the highlighted passages, giving a particularly unpleasing appearance. Furthermore, the tip of the highlighter is also polluted to some extent by the dyes that it has partially dissolved from the ink of the written text.